The present invention relates to a brazing alloy and a brazing material for stainless steel, which are to be used for braze-joining stainless steel components to each other or for braze-joining a stainless steel component to an iron or a steel component. The invention further relates to a structure braze-assembled with the use of such a brazing alloy. Examples of the braze-assembled structure include heat exchangers such as radiators and gas coolers, chemical apparatuses, and composite pipes having a stainless steel sleeve.
With an internationally growing interest in environmental issues, there has recently been an increasing demand for reduction in automobile emission, which is typified by mandatory introduction of zero emission vehicles (ZEVs) which is to come into effect in the State of California, the United States. As part of measures for automobile emission cleaning, a variety of cleaning apparatuses such as catalytic converters and thermal reactors for converting CO and HC into CO2 and H2O by re-combustion of emission gas have already been put into practical applications.
A conventional heat exchanger for use in such an emission gas cleaning apparatus is generally composed of a stainless steel, such as SUS304 or SUS430 specified by JIS (Japanese Industrial Standard), which is highly corrosion-resistant. Components of the heat exchanger are braze-joined to each other with a copper brazing filler or a copper-based brazing alloy which consists of 5 to 20% of Mn, optional 1 to 5% of Ni, and the balance of Cu, as disclosed in Japanese Unexamined Patent Publication No. 60-72695 (1985).
With recent improvement and development of thermal reactor systems, however, heat exchangers thereof are subjected to a severer corrosive environment due to changes in the composition of the emission gas and the like. This results in such problems as exfoliation of an oxide scale due to repeated heat cycles during use and corrosion by emission gas condensate.
Therefore, a brazing alloy for use in assembling of such a heat exchanger is required to be more resistant to corrosion and oxidation.
The corrosion- and oxidation-resistant requirements are not directed only to the automobile emission gas cleaning systems. The resistance to corrosion and oxidation is required not only for various emission gas heat exchangers for gas turbine engines and the like, but also for chemical apparatuses for use in corrosive and oxidative environments and for iron or steel structures comprising stainless steel components braze-joined to a desired portion thereof to be exposed to such environments.
It is therefore an object of the present invention to provide a brazing alloy for stainless steel which provides a braze-joint having excellent corrosion- and oxidation-resistance, to provide a structure braze-assembled with the use of such a brazing alloy, and to provide a brazing material for stainless steel which ensures higher productivity.
A brazing alloy for stainless steel according to the present invention is a brazing alloy which is to be used for braze-joining a stainless steel component to a component composed of iron or steel including stainless steel. The brazing alloy consists essentially of Mn, Ni and Cu, and has a composition in terms of weight percentage which, when plotted on a diagram as shown in FIG. 1, falls within a range defined by:
the point A (37% Mn, 63% Ni, 0% Cu);
the point B (18% Mn, 27% Ni, 55% Cu);
the point C (42% Mn, 3% Ni, 55% Cu);
the point D (50% Mn, 3% Ni, 47% Cu); and
the point E (50% Mn, 50% Ni, 0% Cu), wherein Mn=50% is exclusive. For convenience, weight percentages will often be represented simply by xe2x80x9c%xe2x80x9d.
Various studies have been made on the composition of an alloy comprising Cu, Ni and Mn as essential elements to impart the alloy with excellent corrosion- and oxidation-resistance and a melting point equivalent to that of a copper brazing filler and, as a result, it has been found that the alloy having the composition which falls within the range defined by the points A, B, C, D and E in FIG. 1 (wherein Mn=50 wt % is exclusive) has the aforesaid properties. More specifically, the Cu and Ni contents should be Cuxe2x89xa655% (preferably, Cu less than 50%) and Nixe2x89xa73.0%, respectively, for excellent corrosion-resistance. If the Mn content is too high, the oxidation-resistance is deteriorated. Therefore, the Mn content should be Mn less than 50% for excellent oxidation resistance. In order to impart the brazing alloy with a melting point of 1100 to 900xc2x0 C. so that diffusion of Fe from the stainless steel into the brazing alloy can be suppressed at brazing and so that a structure having a stainless steel component and an iron or a steel component can be braze-assembled simultaneously with annealing of these components, the Ni content should be lower than a line A-B in FIG. 1, because an alloy having a Ni content higher than the line A-B has a melting point of higher than 1100xc2x0 C. A line A-E in FIG. 1 represents a composition range where the Cu content is Cu=0%. The inventors have found that this range is inclusive in the composition range of the brazing alloy for stainless steel.
In accordance with the invention, an element other than the essential elements Cu, Ni and Mn which does not deteriorate the corrosion- and oxidation-resistance and the low melting point property of the brazing alloy in the aforesaid composition range but even improves these properties can be added to the brazing alloy. For example, one or more elements selected from the group consisting of not greater than 5.0% of Cr, not greater than 5.0% of Co, not greater than 5.0% of Mo, not greater than 5.0% of Al and not greater than 3.0% of Ti may be present in a total amount of not greater than 5.0% in the brazing alloy. Further, not greater than 2.0% of Si may be present in the brazing alloy either alone or along with the aforesaid element such as Cr.
A braze-assembled structure according to the present invention comprises a stainless steel component and a component composed of iron or steel including stainless steel, wherein the stainless steel component and the component are braze-joined to each other with the aforesaid brazing alloy for stainless steel. The braze-assembled structure is highly corrosion- and oxidation-resistant as a whole, because its braze-joint is composed of the aforesaid brazing alloy thereby to have excellent corrosion- and oxidation-resistance.
A brazing material for stainless steel according to the present invention is a brazing material which is to be used for braze-joining a stainless steel component to a component composed of iron or steel including stainless steel. The brazing material comprises: at least one Mn-based metal layer which is composed of a Mnxe2x80x94Ni alloy consisting essentially of Mn and Ni or a Mnxe2x80x94Nixe2x80x94Cu alloy consisting essentially of Mn, Ni and Cu; and at least one Ni-based metal layer which is composed of Ni, a Ni alloy consisting essentially of Ni, or a Nixe2x80x94Cu alloy consisting essentially of Ni and Cu. The Mn-based metal layer and the Ni-based metal layer are bonded to each other in a stacked relation, and an average content of each of the elements in any cross section perpendicular to the length of the brazing material falls within the composition range of the brazing alloy.
The brazing alloy of the present invention generally has a bad workability. Particularly, the brazing alloy, if having a composition which falls within a range defined by the point A, the point F (23% Mn, 37% Ni, 40% Cu), the point G (35% Mn, 25% Ni, 40% Cu), the point H (50% Mn, 45% Ni, 5% Cu) and the point E in the aforesaid composition range in FIG. 1, is difficult to work. In accordance with the invention, the Mn-based metal layer and the Ni-based metal layer of the brazing material can be composed of a Mnxe2x80x94Ni or Mnxe2x80x94Nixe2x80x94Cu alloy, and Ni or a Ni or Nixe2x80x94Cu alloy, respectively, which are commercially available as general purpose metal materials and generally have a good workability. Since the average content of each of the elements in any cross section perpendicular to the length of the brazing material falls within the composition range of the brazing alloy, the Mn-based metal layer and the Ni-based metal layer are melted together to provide the aforesaid brazing alloy composition when the brazing material fuses at brazing. This brazing material is neither in a powdery form nor in a scale form but has a definite shape, thereby ensuring a satisfactory brazing operability. The brazing material ensures a high productivity unlike the brazing alloy of the present invention having a bad workability, because there is no need to cast the brazing alloy and shape a piece of the cast alloy into a proper form such as a linear form.
Another brazing material according to the present invention is a brazing material for stainless steel which is to be used for braze-joining a stainless steel component and a component composed of iron or steel including stainless steel. The brazing material comprises a Mn-based metal layer which is composed of a Mnxe2x80x94Ni alloy consisting essentially of Mn and Ni or a Mnxe2x80x94Nixe2x80x94Cu alloy consisting essentially of Mn, Ni and Cu, and two Ni-based metal layers which are each composed of Ni, a Ni alloy consisting essentially of Ni, or a Nixe2x80x94Cu alloy consisting essentially of Ni and Cu. The Ni-based metal layers are stacked on opposite sides of the Mn-based metal layer and press-bonded thereto, and an average content of each of the elements in any cross section perpendicular to the length of the brazing material falls within the composition range of the brazing alloy.
Since the Ni-based metal layers are more corrosion-resistant than the Mn-based metal layer, the brazing material of the invention has outer surfaces less susceptible to corrosion with the least number of layers including the Mn-based metal layer and the Ni-based metal layers. This prevents deterioration in the quality of the brazing material and ensures easy handling. Since the Ni-based metal layers are press-bonded to the Mn-based metal layer, the brazing material of laminate structure can easily be produced. This ensures a higher industrial productivity, and reduces production costs. The press-bonding can easily be achieved by stacking the Ni-based metal layers on the Mn-based metal layer and passing the resulting stack between a pair of press rolls.